Various rubber compositions for tire treads in which an inorganic filler such as silica is added to a diene-based rubber such as natural rubber and/or synthetic rubber used as the rubber component have been proposed so that suppressed fuel consumption and excellent gripping property on wet roads are simultaneously exhibited.
When the bonding strength in a rubber composition using an inorganic filler such as silica is compared with that of conventional rubber compositions using carbon black, the bonding between the inorganic filler and the rubber component is smaller than the bonding between carbon black and the rubber component.
In the rubber composition using silica or the like, in general, a silane coupling agent which is an organosilicon compound is used for enhancing the bonding between the inorganic filler and the rubber component. The silane coupling agent enhances the bonding between the inorganic filler and the rubber component and improves abrasion resistance of a tire tread. In particular, silane coupling agents which are organosilicon compounds having a combination of an alkoxysilyl group and sulfur in the molecule are preferable since the inorganic filler such as silica and the rubber component are easily bonded to each other by the vulcanization reaction.
However, the silane coupling agents which are organosilicon compound having an alkoxysilyl group in the molecule has a problem in that an alcohol is formed in the reaction with the inorganic filler such as silica. Foams are formed at the inside of an unvulcanized rubber obtained after extrusion since the formed alcohol is vaporized in the rubber, and dimensional stability and productivity decrease. It is desired that discharge of volatile alcohols is decreased from the standpoint of the health of the producer and the consumer and the consideration on the environment, also.
To overcome the above problem, various coupling agents which are organosilicon compounds suppressing discharge of alcohols have been proposed (Patent References 1 to 3). However, the suppressed discharge of volatile alcohols and the sufficient reactivity with an inorganic filler such as silica are not achieved simultaneously to the desired degree by using the above compounds.
[Patent Reference 1] Japanese Patent Application Laid-Open No. 2002-275311
[Patent Reference 2] Japanese Patent Application Publication (Tokuhyo) No. 2005-500420
[Patent Reference 3] Japanese Patent Application Laid-Open No. 2005-35889
[Patent Reference 4] Japanese Patent Application Laid-Open No. 2000-103794